The vessels of plant xylem consist of tubular cells which undergo programmed cell death, leaving long thin tubes which conduct water to the top of even the tallest tree, through capillary action. As illustrated below, the individual xylem vessel elements include perforation plates at their ends; it is these which can act as water filters.
Lee, Boutilier, Chambers, Venkatesh, and Karnik found that the water filter illustrated above could remove particles larger than 0.1 µm – enough to filter out bacteria, though not viruses. Epoxy glue is needed to seal the filtering wood into tubing, however, and the flow rate is a low 180 ml per hour, even under pressure. Still, this is a very interesting low-cost water-purification technique.